An apparatus for mixing and dispensing liquid resins is widely employed to mold various plastic products which are usually some type of polymers produced by some type chemical reaction such as polymerization carried out predominantly in a mold employing some type of monomers or mixtures of monomers and hardening agents such as bridge formation agents.
Historically, monomers in the form of powder or particles were predominantly utilized. Recently, however, liquid monomers have become preferably utilized. This tendency has revised the technical requirements for such an apparatus for mixing and dispensing resins, because liquid monomers are inclined to allow chemical reaction such as polymerization to commence immediately after monomers are mixed with bridge formation agents.
It is widely known that injectors or extruders have been employed as the apparatus for mixing and dispensing resins in the form of powder or particles. Due to the aforementioned additional requirements caused by the nature of the liquid monomers, the apparatuses for mixing and dispensing resins applicable to liquid resins are required to be provided with additional mixing function. Therefore, the apparatuses for mixing and dispensing liquid resins presently available in the prior art are generally classified into four categories including an advance mixing system, a pre-mixing chamber system, a deformed screw system and a static mixing system.
An advance mixing system is a system which consists of a dispensing unit such as an injector or an extruder and a mixing unit which is installed apart from the dispensing unit and has a function to mix a resin with a hardening agent. In other words, this system consists of two entirely independent units including a mixing unit and a dispensing unit. A resin and a hardening agent are mixed in the mixing unit, before the mixed materials are conveyed and charged to the dispensing unit for the purpose of being dispensed into a mold. In this sense, this system is a type of batch system. Since the hardening reaction commences in the mixing unit, this system is involved with difficulties in conveyance of the mixed liquid resins and in quality control of the molding of plastic products. It is noted, however, the quantity of mixing units can be selected independently from that of dispensing units.
A pre-mixing chamber system is a system which consists of a mixing unit which is a type of agitator and a dispensing unit which usually is an in-line screw injector, both of which are driven by a common drive. Albeit this system could provide a sizable magnitude of operational advantages, this system does not necessarily provide a sufficient magnitude of functional improvement.
A deformed screw system is an improved type of an in-line screw injector. Such an improvement is applied to the shape of screw to improve the mixing function of an in-line screw injector. Since this system is not provided with an independent mixing unit but is an in-line screw injector itself, the structure is considerably simplified in comparison with the pre-mixing chamber system. One of the exemplary models included in this category is disclosed in the U.S. Pat. No. 4,134,688 which is directed to an improved structure of an in-line screw injector. More specifically, the full-flight screw employed in this improved in-line screw injector is provided with an additional mixing unit to improve the kneading function.
A static mixing system is a system which does not have a movable element. In other words, as disclosed by Armeniades et al. in the U.S. Pat. No. 3,286,992, this system consists of a hollow tube containing a series of curved dividing elements each of which has a shape resembling to the fin of a fish. A pump is employed to flow a mixed liquid resins in the tube along the surfaces of curved dividing elements. Due to the three independent functions including flow division, radial mixing and flow inversion, the mixed liquid resins are entirely mixed, before it is dispensed into a mold. Since this system has no movable element, it is clear that a considerable magnitude of advantages is expected from various viewpoints.
However, I have discovered that even this system is involved with a drawback which occurs at the beginnings of each dispensing action which rather frequently occurs. During the short period in the order of several 100 milliseconds at the beginning of each dispensing action, this system is charged with an unmixed material, a material having a relatively lower viscosity, specifically a resin hardening agent. Since no mixing function is expected for this system unless plural materials are supplied simultaneously, this means a lump of an unmixed material is dispensed into a mold at an unpresumable point of time during a dispensing action. This of course causes an unsatisfactory molding quality for a produced plastic product. Particularly because a continuous operation is not necessarily realistic for this system and because it is difficult to presume the location where such molding faults appear in a plastic product, this is a drawback which cannot be ignored.